service appoinment Automatic filling of Scheduled End or Duration

Hi,

I'm working on CRM 2013.

I made some modifications on service acativity form.

if a user has  security role 'PARTNER', then all fields on service activity are read only except Scheduled Start. 

Now the problem is, when user sets Scheduled Start and the Scheduled Duration Minutes is 1hour, the Scheduled End is not automatically modified. Its remains the same because this field is disabled

Does any one knows where to find the code that autofills Scheduled End field?

Or any suggestions to handle this issue is appreciated.